feat(vue): add loading and error slots to FormRelay component

[RobertBoes]

Summary

• Adds optional #loading and #error named slots to the <FormRelay> component
• #loading renders while the schema is being fetched — prevents showing an empty form with a visible submit button
• #error renders when the schema fetch fails, with { error } as slot props
• Fully backwards compatible: omitting the slots preserves existing default-slot behavior (schemaLoading/schemaError still passed as props)

Usage

<FormRelay form-id="..." public-key="...">
  <template #loading>
    <p>Loading form...</p>
  </template>
  <template #error="{ error }">
    <p>Failed to load form: {{ error.detail }}</p>
  </template>
  <template #default="{ fields, submit, canSubmit }">
    <!-- form fields + submit button -->
  </template>
</FormRelay>

Test plan

• Renders loading slot while schema is fetching
• Switches to default slot after schema loads
• Falls through to default slot (with schemaLoading: true) when no loading slot provided
• Renders error slot when schema fetch fails
• Falls through to default slot (with schemaError) when no error slot provided
• All existing tests continue to pass